Added gradients interactive to test gradients

Again, the theme is Chrome.
Added gradients parser

This is the parser for the gradient syntax I plan to use for md2d. Try
it out!
Fixes and improvements to gradients.html and gradients.js
Gradients can now be interpreted from the MML
Also, getting colors is now more streamlined with getColorProperty.
Gradients can now render
Obstacles can now have gradients
The obstacle color system has been rewritten to match the shape color system.
Tests now pass, minor fixes done
The updated obstacle color system was causing mismatches with the expected JSON.
That's quite expected!
Fixed missing function in gradient.html